The invention relates to color interpolation.
FIG. 1 shows a semiconductor imager 10 (e.g., a complementary metal-oxide semiconductor (CMOS) imager) might be used to electrically capture xe2x80x9csnapshotsxe2x80x9d of an optical image. The imager is used to convert an optical image into an electrical representation. The imager 10 accomplishes this conversion through the use of an array of sensing elements arranged as pixel cells 12 that sense the intensity of light coming from the image. The xe2x80x9cexposure timexe2x80x9d for each snapshot depends on an integration interval during which each pixel cell 12 integrates an indication of the number of photons of light striking the cell 12 (i.e., measures an intensity of light striking the cell 12) and provides an indication of the integrated value via an analog output signal. For CMOS imagers, on-chip analog conditioning circuitry 14 (e.g., circuitry to perform correlated double sampling and gain control) and an analog-to-digital converter (ADC) 16 process the analog outputs of the pixel cells 12 to provide a digital representation of the image which can be retrieved from the imager 10 through a parallel port interface 18.
The pixel cells 12 provide an indication of the intensity of light striking the cell 12. Hence, the above-described arrangement may be used to produce a monochrome or luminance only representation of the image. However, to produce color representations of the image, the imager also needs to provide information about primary colors (e.g., red, green and blue colors) of the image. To accomplish this, each pixel cell 12 is configured to sense the intensity level of light in one of the primary color bands. A typical way to accomplish this is to cover each pixel cell 12 with a spectrum-discriminating filter (e.g., a filter that only allows a red, green or blue color band to pass through the filter). As a result, some pixel cells 12 sense red light, some pixel cells 12 sense green light and some pixel cells 12 sense blue light. As an example, a multi-band filter pattern 20 (see FIG. 2) placed over the array of pixel cells 12 may have alternating red, green and blue filter stripes that extend along the columns of the array. Thus, each filter stripe of the pattern 20 configures one of the columns of the array to sense light in one of the primary color bands. As another example, the filter pattern may be checkered, instead of striped.
Each pixel cell 12 captures a portion of the image. To maximize the resolution of the image when reproduced on a display, it is desirable to form a one-to-one correspondence between the pixel cells 12 of the imager 10 and pixels of the display. However, with color imagers, three adjacent pixel cells 12 (each pixel cell 12 sensing a different primary color band) are typically used to provide the information needed to form one pixel on the display. Thus, when used to capture color images, the effective display pixel resolution of the imager 10 typically is one third of the actual pixel cell 12 resolution.
For purposes of preserving a one-to-one correspondence between the pixel cells 12 and the pixels of the display, one solution is to form an imager having three times as many pixel cells as corresponding pixels of the display to compensate for the three primary colors. Referring to FIG. 3, another solution is to use three imagers 22, 24, and 28, one for each primary color band of the image. Thus, for example, one imager 22 (covered by a red filter) senses red light, one imager 24 (covered by a green filter) senses green light, and one imager 26 (covered by a blue filter) senses the blue light coming from the image. Dichroic plates 28 may be used to split the light into beams into its primary colors.
Referring to FIG. 4, a third solution might be to use an off chip discrete-time signal processing (DSP) engine 30 to interpolate the two missing colors for each pixel cell 12. To accomplish this, the DSP engine 30 processes the color information provided by adjacent pixel cells 12. Typically, nearest neighbors are weighted with predetermined coefficients and averaged to determine a color at a particular pixel cell location. For example, referring back to FIG. 1, a pixel cell 12a that is covered by a red filter provides a representation of a red color of the portion of the image striking the cell 12a. To ascertain the blue color of the portion of the image otherwise striking the cell 12a (if not for the red filter), the DSP engine 30 averages (a weighted representation of) the outputs of adjacent pixel cells 12b and 12c (i.e., adjacent pixel cells covered by a blue filter) to interpolate the missing blue color. The DSP engine 30 also interpolates the green color of the portion of the image that would other strike the cell 12a in a similar manner.
In general, in one aspect, the invention features an imager that has first and second photosensitive sites and an interpolator located in a semiconductor substrate. The first photosensitive site is configured to receive light having a spectral component, and the second photosensitive site is configured to measure the level of the spectral component in light received by the second photosensitive site. The interpolator is configured to estimate the level of the spectral component in the light received by the first photosensitive site based on the measurement by the second photosensitive site.
Implementations of the invention may include one or more of the following. The first and/or second photosensitive sites may include a pixel cell and a filter that covers the pixel cell. The filter covering the first photosensitive site may be configured to prevent the spectral component from striking the pixel cell, and the filter covering the second photosensitive site may be configured to allow the spectral component to strike the pixel cell. The first photosensitive site may also be configured to measure the level of another spectral component in light received by the first photosensitive site, and the interpolator may be also configured to estimate the level of another spectral component in the light received by the second photosensitive site based on the measurement by the first photosensitive site.
The imager may also include a third photosensitive site (also located in the substrate) that is configured to measure the level of the other spectral component in light received by the third photosensitive site. The first photosensitive site may also be configured to receive light having the another spectral component, and the interpolator may also be configured to estimate the level of the spectral components in the light received by the first photosensitive site based on the measurements by the second and third photosensitive sites.
In general, in another aspect, the invention features an imager that has first and second photosensitive sites and an interpolator located in a semiconductor substrate. Each first photosensitive site is configured to receive light having a spectral component, and each second photosensitive site is configured to measure the level of the spectral component in light received by the second photosensitive site. The interpolator is configured to estimate the level of the spectral component in the light received by at least one of the first photosensitive sites based on the measurements by the second photosensitive sites.
Implementations of the invention may include one or more of the following. The interpolator may include an averaging circuit that is configured to perform the estimation by averaging some of the measurements by the second photosensitive sites. The interpolator may also include a scaling circuit that is configured to scale some of the measurements by predetermined coefficients before being averaged by the averaging circuit. The scaling circuit may be programmable to change one or more of the coefficients. The first and second photosensitive sites may be part of an array of photosensitive sites (e.g., located in a column of the array, a row of the array, or arranged in a rectangular block of an array).
In general, in another aspect, the invention features a color imager for use with light having first, second and third primary color bands. The imager has first, second and third photosensitive sites and an interpolator located in a semiconductor substrate. Each first photosensitive site is configured to receive a portion of the light and measure a level of the first primary color band in the portion of light received by the first photosensitive site. Each second photosensitive site is configured to receive a portion of the light and measure a level of the second primary color band in the portion of light received by the second photosensitive site. Each third photosensitive site is configured to receive a portion of the light and measure a level of the third primary color band in the portion of light received by the third photosensitive site. The interpolator is configured to estimate the levels of the second and third primary color bands in the light received by the first photosensitive sites based on the measurements by the second and third photosensitive sites; estimate the levels of the first and third primary color bands in the light received by the second photosensitive sites based on the measurements by the first and third photosensitive sites; and estimate the levels of the first and second primary color bands in the light received by the third photosensitive sites based on the measurements by the first and second photosensitive sites.
Implementations of the invention may include one or more of the following. The interpolator may be also configured to furnish a representation of the levels of the first, second and third primary color bands for each of the first, second and third photosensitive sites. The representation for each site may include a representation (e.g., a true color representation) of the color of the light received by the site.
In general, in another aspect, the invention features a method that includes using a first photosensitive site located in a semiconductor substrate to receive light having a spectral component. A second photosensitive site located in the substrate is used to measure the level of the spectral component in light received by the second photosensitive site. An interpolator located in the substrate is used to estimate the level of the spectral component in the light received by the first photosensitive site based on the measurement by the second photosensitive site.
In general, in another aspect, the invention features a method that includes using first photosensitive sites located in a semiconductor substrate to receive light having a spectral component. Second photosensitive sites located in the substrate are used to measure the level of the spectral component in light received by each of the second photosensitive sites. An interpolator located in the substrate is used to estimate the level of the spectral component in the light received by at least one of the first photosensitive sites based on the measurements by the second photosensitive sites.
Among the advantages of the invention are one or more of the following. True color imaging occurs on a single semiconductor chip. The pixel cells of the imager and the pixels of the display have a one-to-one correspondence Only one imager is required. The imager may be used with many commonly used color filter patterns.
Other advantages will become apparent from the following description and from the claims.